The present invention relates to the electrochemical treatment of aluminum surfaces. More particularly, the invention relates to treatments for an aluminum substrate to provide it with a surface which has improved adhesion to subsequently applied coatings which are useful for photographic elements in lithography. Such surfaces are also useful for capacitors, dielectric applications and other applications where a barrier layer is useful.
The use of aqueous electrolytes to produce an anodized aluminum surface for lithographic printing has been known for many years, and many methods of manufacture are known to those skilled in the art.
Although phosphoric acid anodized layers have many desirable features for lithographic use, it is well known that the anodized film is of a rather soft nature and of relatively poor abrasion resistance. This is a disadvantage when long run printing performance is required. Sulphuric acid anodized layers are much harder, have much greater abrasion resistance and therefore provide more robust lithographic printing plates capable of very long printing runs. They have consequently enjoyed and continue to enjoy considerable commercial success. The sulphuric acid process is also more easily adapted to producing thicker anodized layers than the phosphoric acid process, the latter tending to be self limiting owing to the greater solubility of the anodized film in the electrolyte. However, the sulphuric acid process produces layers which are not entirely satisfactory as lithographic substrates, since their receptivity towards ink and water tends to be non-discriminatory and methods of overcoming ink receptivity in the non-printing areas for example have to be used for all, but the thinnest of layers. This necessity for "desensitizing" the anodized layer can produce a serious problem, for example when it is required to manufacture pre-sensitized lithographic plates in which the adhesion of the light sensitive coating to the substrate during and after development is all important. When such densensitizing treatments are made to the anodized layer (for example by "sealing" or by "snubbing") the attainment of satisfactory ink-repellency in the non-printing areas, and at the same time satisfactory performance in the ink-receptive image areas, is a matter of well judged compromise. Such treatments can give lithographically desirable results and are in fact well known and in commercial use for both phosphoric acid and sulphuric acid anodized aluminum lithographic printing plates.
Examples of aluminum anodization in aqueous acidic electrolytes may be found in U.S. Pat. Nos. 3,900,370; 3,558,446 and 3,181,461. Aluminum treatments employing polyacids are taught in U.S. Pat. Nos. 4,153,461; 3,136,636 and 4,022,670. However, each disclosure employs aqueous treatment solutions which are conducive to oxide formation on the aluminum surface. The present invention in contradistinction employs non-aqueous electrolytes to form a thin, substantially non-porous organometallic complex on the aluminum surface and no substantial oxide formation. This surface complex demonstrates advantageous adhesive properties to subsequently applied coatings which are useful in the lithographic arts.